1. Field of the Invention
The present invention relates to a recorder, and more particularly to an ink jet printer.
2. Related Background Art
FIG. 1 shows an example of a prior art ink jet printer. A print paper is fed by a line feed motor 7 along a platen 1. When a paper sensor 9 detects the print paper, printing is started.
An auto-cap motor 6 is driven to remove a cap 3 from a print head 2. When a cap sensor 10 detects that the cap 3 has been removed, a carriage motor 8 is driven. The rotation of the carriage motor 8 is transmitted to the printer head 2 by a pulley 4 and a belt 5 so that the print head 2 is moved. The print head 2 receives an ink discharge timing signal from an ink discharge timing detection circuit 16 to discharge ink. The ink discharge timing signal may also be used as a servo signal for the carriage motor 8 to move the print head 2 at a constant velocity. Main scan is effected by the movement of the print head 2, and sub-scan is effected by the rotation of the platen 1.
Numeral 11 denotes a home position sensor for detecting a reference position of print head 2, numeral 12 denotes an ink sensor for detecting presence or absence of ink, numeral 13 denotes an ink tank, and numeral 14 denotes a flexible tube for supplying the ink from the ink tank 13 to the print head 2.
FIG. 2 illustrates ink discharge status at a point A from an orifice of the print head 2 of the prior art printer. The print head 2 reciprocally moves at a velocity of.+-.V (m/sec) at the point A. The ink is discharged at a velocity V (m/sec) from the print head 2 at the point A, perpendicularly to the plane of the print paper. When the print head 2 moves right at the velocity V, the ink flies from the point A to the print paper at a velocity V+V. The ink droplet is deposited on the print paper at a point displaced by l (m) in x-direction. When the print head 2 moves left, the ink flies at a velocity V-V and the ink droplet is deposited on the print paper at a point displaced by -l in x-direction. When a distance L between the print head 2 and the print paper, a magnitude V of the velocity V of the print head 2, that is, a speed V of the print head 2, a magnitude v of the ink discharge velocity V, that is a discharge speed v of the ink are constant, a deviation between the ink deposition points in the reciprocal movement when the ink discharge timing signal is prepared at the point A is equal to 2=2LV/v(m).
FIG. 3 shows a prior art ink discharge timing detection circuit 16. FIG. 4 shows a prior art ink discharge timing pulse generation method. As a slit 16B moves in the x-direction with the print head 2, a light from a light emitting diode 16A is directed to a photo-transistor 16C through an encoder slit 15 and the slit 16B (see FIG. 4(A)). As a result, a signal Vs is produced (see FIG. 4(B)). This signal is compared with a threshold voltage Vth by a comparator 16D which produces a reshaped rectangular wave Vout (see FIG. 4(C)). P in FIG. 4 represents a slit width which may be 0.15 mm. As described above, in the bidirectional printing, there is a deviation of 2 l(m) for the ink deposition point. This deviation is compensated by discharging the ink when the print head 2 is at a point which is l behind the point A shown in FIG. 2.
The above method may work well if a record medium (print paper) of a constant thickness is always used, but if a record medium of different thickness is used, that is, if L varies by .DELTA.L in FIG. 2, the ink deposition point deviates by 2 .DELTA.L/v.multidot.V. As a result, if a record medium such as a post card which is thicker than a normal record medium is used, characters and image are not sharply printed.